1. Field of the Invention
The present invention relates to a beam light scanning apparatus for scanning a beam light, an image forming apparatus equipped with the beam light scanning apparatus, and a beam light scanning method. More particularly, the present invention relates to a beam light scanning apparatus, an image forming apparatus, and a beam light scanning method, which are suitable for, e.g., a copying machine and are capable of adjusting the transfer timing of image data in a transfer path extending until a driver for causing the beam light to be emitted.
2. Description of the Related Art
Recently, various types of image forming apparatuses, such as digital copying machines and laser printers, have been developed and put already into practice in which an image is formed with a combination of scanned exposure using a laser beam light (hereinafter referred to simply as a “beam light”) and an electrophotographic process.
That type of image forming apparatus operates based on the principle that, as disclosed in, e.g., Patent Document 1; Japanese Unexamined Patent Application Publication No. 2001-91872, the surface of a single photoconductor drum is scanned and exposed at the same time using the beam light to form a single electrostatic latent image on the surface of the photoconductor drum, and the electrostatic latent image is transferred to a sheet of paper.
FIG. 10 shows generation of a driving signal applied to a laser driver when the electrostatic latent image is formed on the surface of the photoconductor drum based on the above-mentioned principle by driving a laser oscillator.
As shown in FIG. 10, a horizontal sync sensor 201 detects the passage timing of a scan beam light that is scanned by a polygon mirror (not shown) in the predetermined direction of main scan, to thereby generate a horizontal sync signal BD. The horizontal sync signal BD (hereinafter referred to simply as the “BD signal”) is connected to a PWM circuit 203 of a laser control unit 202. The PWM circuit 203 outputs a line sync signal LSYNC (hereinafter referred to simply as an “LSYNC signal”) in sync with the BD signal to an image data I/F 204.
Upon receiving the LSYNC signal, the image data I/F 204 outputs image data (DAT_IN) as input data to the laser control unit 202 in sync with an image data transfer clock (CLK_PIX) that is in turn in sync with the LSYNC signal.
The PWM circuit 203 receives the image data (DAT_IN) and issues a corresponding PWM output (DAT_OUT) to a laser driver 205. The laser driver 205 employs the PWM output (DAT_OUT) as a driving signal and drives a semiconductor laser 206 in accordance with the driving signal. The semiconductor laser 206 emits a laser beam in the form of pulsated light corresponding to the driving signal, and the beam light is scanned in the direction of main scan by an optical system including the polygon mirror. Accordingly, a latent image is formed per pixel on the surface of a photoconductor drum in the lengthwise direction thereof (i.e., in the direction of main scan). The photoconductor drum is rotated in sync with the scan of the beam light in the direction of main scan. At the next scan timing, therefore, the position of the scan line is moved to a next pixel position in the rotating direction of the photoconductor drum (i.e., in the direction of sub-scan), and formation of a latent image is similarly repeated in sync with the BD signal. As a result, a two-dimensional latent image is mapped on the surface of the photoconductor drum in both the direction of main scan and the direction of sub-scan. That latent image is then printed as an image.
FIGS. 11 and 12 are conceptual and detailed timing charts showing, by way of example, the above-described process of forming the PWM output (DAT_OUT).
A tailing edge of the LSYNC signal is outputted in sync with a leading edge of the BD signal. Also, the relative timing (time interval) Tsync between the leading edge of the BD signal and the trailing edge of the LSYNC signal is always constant. The image data I/F 204 transfers the image data (DAT_IN) to the laser control unit 202 in sync with the image data transfer clock (CLK_PIX) that is in turn in sync with the LSYNC signal (the image data transfer clock essentially serves as an image clock; namely, the frequency of the image clock is an image frequency per pixel). In accordance with the input image data (DAT_IN), the laser control unit 202 outputs an ON/OFF signal (DAT_OUT), i.e., the PWM output, to the laser driver 205 so that the laser 306 emits the beam light.
Thus, because the timings of generation of the BD signal and the LSYNC signal are always constant, the emission of the beam light from the laser is also in sync with the BD signal and an image can be formed without causing a deviation in the main scan.
With a recent tendency toward a “higher printing speed” and “finer image resolution”, speedup of the image clock is demanded in many cases. Trying to speed up the image clock is equivalent to speedup of the transfer clock.
The speedup of the transfer clock faces a problem at the present. For example, the image data I/F 204 and an image processing unit 207 disposed upstream of the former start processing upon receiving the LSYNC signal and transfers the image data (DAT_IN) to the laser control unit 202 after completion of the processing. With the higher speed and the finer resolution (which inevitably lead to a higher rotation speed of the polygon mirror and a higher scan speed of the beam light), however, there may occur a trouble that the above-mentioned processing is not finished in time, whereby proper data cannot be transferred and a desired image cannot be formed.
One conceivable solution for overcoming such a problem is to execute the processing in the image processing unit 207 and the image data I/F 204 in advance, and to store the processed data in storage means in units of line (called a line memory). However, the line memory is expensive and the use of the line memory gives rise to another problem of increasing the parts cost of the apparatus. In the case using line memories for several lines, particularly, the parts cost is further increased correspondingly.